As a process for preparing a 3-(substituted phenyl)-1,3-oxazolidine-2,4-dione derivative, the process which comprises reaction of an N-arylcarbamate with a 2-hydroxyalkanoate, if necessary, in the presence of a catalyst under heating [EP-A-0004582 (U.S. Pat. No. 4,220,787)] has been known so far. However, compounds having an alkylidene group at the 5-position can not be prepared by the process.
As a process for preparing a 3-(substituted phenyl)-5-alkylidene-1,3-oxazolidine-2,4-dione derivative, the process which comprises reaction of a (substituted phenyl) isocyanate with a 2-hydroxy-3-alkenoate in the presence of a base followed by cyclization and isomerization of the double bond [WO 87/02357 (EP-A-0241559, U.S. Pat. No. 4,818,272, U.S. Pat. No. 4983751)] is known. However, the process is not thoroughly satisfactory as an industrial process because it is necessary to use phosgene or phosgene dimer, which is extremely dangerous to handle, for preparation of the isocyanate used as a starting material in this process.